A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning object, which is alternatively referred to as a mask reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning object to the substrate by imprinting the pattern onto the substrate.
A conventional lithographic apparatus, typically comprises a support structure (e.g., a mask table) that is provided and constructed to support the patterning object. Conventionally, the patterning object (e.g., a mask) is clamped by vacuum to the support structure.
In a conventional mask clamp configuration, for example, the mask is preloaded by vacuum to contact areas of a vacuum clamp of the mask table. The contact areas comprise an outer edge, an inner edge and a collection of burls. Each contact area defines a local contact area. At each local contact area, a part of the inertial force to the mask is countered by friction. The amount of friction to be countered for each local contact area is limited. The frictional limit depends on various parameters, such as normal force, contamination level, humidity, material combination, surface roughness, etc. Using a spring-mass model in the scan direction of the vacuum clamp, each local contact area may experience a different stiffness to the mask table. Further, the pre-load, or normal force, may also be different for each local contact area. As a result, at certain mask table acceleration levels, an overlay error may be introduced. Because of increased inertia-forces to the mask, the mask may show a non-reproducible (i.e. incorrectible) micro-slip behavior (hysteresis) resulting in an overlay error. The hysteresis is caused by local slip of the mask at the contact areas. For example, at a certain acceleration level of the mask table, the mask slips at the outer edge while it sticks to the burls and the inner edge. This results in a displacement of the mask when the acceleration phase of the mask table is over and illumination starts.